Zoologger: Sea slugs stab partners in head during sex

Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species:Siphopteron species 1Habitat: warm and shallow waters off the coast of Australia

Slowly, almost tenderly, the two lovers intertwine their bodies. After a long embrace they gently begin copulating. Then, a few seconds later, they stab each other in the head.

If this doesn't sound like a particularly nice way to spend a Saturday night, relax: the mating couple in question are sea slugs. They are one of many animals that inject chemicals into their partners while mating, but the only known species that consistently aims for the head.

It's not clear why these sea slugs stab each other, but it may be a form of mind control. By injecting chemicals directly into its partner's brain, each sea slug may be able to change the other's behaviour to its own advantage.

Colourful molluscs

There are hundreds of species of sea slug, but the head-stabbing ones belong to a genus called Siphopteron. The species has not been formally described, and until that happens it is simply called "Siphopteron species 1". They are tiny compared with most of their relatives, just 2 to 4 millimetres long, and live on sand beds in shallow water.

Like many sea slugs, all Siphopteron species are simultaneous hermaphrodites, and so every one of them has male and female sexual organs. When two slugs mate, each prefers to act as a male, because that way they can fertilise lots of other slugs.

When they finally get down to business, the mating isn't pretty. Each slug has a jelly-like tube with a sharp tip called a penile stylet that punctures the other's skin, and then injects a mix of secretions made by the prostate gland.

In Siphopteron quadrispinosum, the closest relative of Siphopteron species 1, these injections are beneficial to the receiver, despite causing superficial damage. As well as chemicals, the secretions may contain nutrients: if that's so, the slugs are giving presents to their mates, albeit in an unconventional way.

Aim for the head

While other Siphopteron species, including S. quadrispinosum, stab fairly indiscriminately anywhere on their partner's body, Rolanda Lange of the University of Tübingen in Germany noticed that Siphopteron species 1 was oddly consistent in its stabbing practices.

She watched 16 copulations in the lab. Pairs of slugs intertwined their bodies, often biting each other's rear ends. After about 15 minutes, each one inserted its penis into the other's genital opening. Seconds later, each slug stabbed the other in the head with its penile stylet, each time going for the spot right next to the eyes. The pair then stayed locked together for about 40 minutes.

In other animals, such as snails and other sea slug species, injections can help to ensure that the male's sperm is used to fertilise eggs, says Joris Koene of the Free University in Amsterdam, the Netherlands. "Some animals can reject sperm, they squirt it back out. And hermaphrodites often have specific organs to digest excess sperm." Koene thinks the chemicals in the injections could deactivate the other partner's rejection mechanism, ensuring that the slugs put the sperm to its intended use.

Brain control

But that doesn't explain why these sea slugs aim for the head. Sea slugs don't have bones, or much internal structure, so in theory it shouldn't matter where the hormones get injected, says Koene. "Wherever you inject something through the skin, you end up in the body cavity."

As well as species 1, Lange also examined four other Siphopteron species. One always stabbed near the genital opening, while the others stabbed at random. None went for the head.

That suggests Siphopteron species 1 is doing something different to its relatives: perhaps targeting its partner's brain to change its behaviour. Koene says the injection might make the other sea slug reluctant to mate. "If you are able to prevent your partner from mating again, your sperm has a longer time to fertilise the eggs," he says.

The Japanese land snail and humble fruit fly take this approach but without the extreme delivery method – the chemicals simply get transferred from one partner to another during mating.

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